The use of liquid crystal materials to exhibit electro-optical effects in display devices such as digital calculators, watches, meters and simple word displays is now well known. However, known liquid crystal materials are not ideal in all respects and a considerable amount of work is currently being carried out in the art to improve their properties. Liquid crystal materials normally consist of mixtures of compounds; improved materials are obtained by forming new mixtures having an improved combination of properties.
Although liquid crystal materials normally consist mainly of compounds which exhibit a liquid crystal phase by themselves, the materials may contain components which do not exhibit such a phase. Compounds forming such components exhibit a virtual or monotropic liquid crystal to isotropic liquid transition (clearing point) at a temperature below their melting point. As is well known to those skilled in the art, monotropic or virtual transitions may be detected respectively by rapid cooling of the liquid phase or by dissolving the compound in a material exhibiting a liquid crystal phase, observing the change in the transition to the isotropic liquid phase of the material by the addition and calculating the virtual transition temperature by extrapolation of the data from a series of such mixtures of known composition.
Compounds which do not exhibit a liquid crystal phase by themselves are useful as additives to liquid crystal materials, eg to improve the liquid crystal temperature range (ie the range over which the material exhibits a liquid crystal phase) and/or to improve the viscosity of the liquid crystal material.
The liquid crystal temperature range of a material is important because it determines the operating temperature range of the display device. This range is desirably as great as possible.
The viscosity of a liquid crystal material is important because it determines the speed of response of the display device, ie the times required to switch the display from the off state to the on state and vice versa. The viscosity is desirably as low as possible. The viscosity of a mixture of compounds forming a liquid crystal material is determined by the viscosity of the individual compounds.
Strictly speaking, the response times are dependent on a number of viscosity coefficients but the main coefficient to be considered is that known as the "flow aligned" viscosity coefficient (see for example the article entitled "Flow aligned viscosities of Cyanobiphenyls" by J. Constant and E P Raynes Mol. Cryst. Liq. Cryst. (1980) Vol 62 pages 115-124). The term "viscosity" as used in this specification is to be understood to mean the flow aligned coefficient in the nematic liquid crystal phase (mesophase) unless otherwise specified.
Compounds normally incorporated in liquid crystal materials for electro-optical applications may generally be represented in their simplest generalised form by formula A as follows: EQU X--Y--X' A
where X and X' indepedently represent aromatic or alicyclic ring structures and Y represents a simple bridging group such as CO.O or a direct carbon-carbon bond.
Formula A normally applies to such compounds whether or not they exhibit a liquid crystal phase by themselves.
The groups X and X' may for example include a 1,4-disubstituted benzene ring, a trans-1,4-disubstituted cyclohexane ring, or a 1,4-disubstituted bicyclo(2,2,2)octane ring. Known compounds of the type which include a bicyclo(2,2,2)octane ring include known compounds of formulae B and C as follows: ##STR3## where R and R' are alkyl groups and Q is hydrogen or fluorine. Compounds of formulae B and C are described in published UK Patent Applications Nos. 2027027A and 2063250A for example.
We have now discovered that a class of bicyclo(2,2,2)octane compounds which surprisingly are not of the generalised formula A are useful as additives in liquid crystal materials.